Semiconductor device and method for producing semiconductor device

ABSTRACT

Hydrogen atoms and crystal defects are introduced into an n− semiconductor substrate by proton implantation. The crystal defects are generated in the n− semiconductor substrate by electron beam irradiation before or after the proton implantation. Then, a heat treatment for generating donors is performed. The amount of crystal defects is appropriately controlled during the heat treatment for generating donors to increase a donor generation rate. In addition, when the heat treatment for generating donors ends, the crystal defects formed by the electron beam irradiation and the proton implantation are recovered and controlled to an appropriate amount of crystal defects. Therefore, for example, it is possible to improve a breakdown voltage and reduce a leakage current.

BACKGROUND OF THE INVENTION A. Field of the Invention

The present invention relates to a semiconductor device and a method forproducing a semiconductor device, and more particularly, to asemiconductor device, such as a diode or an insulated gate bipolartransistor (IGBT) including an n-type field stop layer, and a method forproducing a semiconductor device.

B. Description of the Related Art

As a semiconductor device used in a power semiconductor device, forexample, there is a diode or an IGBT with a breakdown voltage of 400 V,600 V, 1200 V, 1700 V, 3300 V, or a higher one. The diode or the IGBT isused in a power conversion apparatus such as a converter or an inverter.The power semiconductor device requires good electrical characteristics,such as low loss, high efficiency, and a high breakdown voltage, and lowcosts. For example, a semiconductor device has been known in which adonor layer which will be an n-type field stop (FS) layer is provided inan n⁻ drift layer to improve switching characteristics. Thesemiconductor device including the n-type FS layer according to therelated art will be described using a diode as an example.

FIG. 6 is a cross-sectional view illustrating a main portion of thediode including the n-type field stop layer according to the relatedart. In diode 100 a shown in FIG. 6, p-type anode region 2 is formed ina first main surface (front surface 1 b) of n⁻ semiconductor substrate 1which will be an n⁻ drift layer that is so thin that a predeterminedbreakdown voltage is obtained. N⁺ cathode layer 3 is formed in a secondmain surface (rear surface 1 a) of n⁻ semiconductor substrate 1. Then, aplurality of p-type layers and a metal electrode coming into contactwith the p-type layers which form junction edge termination structure 4are formed in the outer circumference of p-type anode region 2 on thefront surface 1 b of n+ semiconductor substrate 1 so as to surroundp-type anode region 2.

Reference numeral 5 denotes an anode electrode, reference numeral 6denotes a cathode electrode, reference numeral 8 denotes an insulatingfilm, and reference numeral 9 a denotes an n-type FS layer. A donorlayer denoted by reference numeral 18 a forms n-type FS layer 9 a.N-type FS layer 9 a is an n-type diffusion layer which has an impurityconcentration higher than n⁻ drift layer 1, has a high impurityconcentration peak at a relatively deep position (for example, at adepth of 3 μm to several tens of micrometers) in the n⁻ drift layer fromrear surface 1 a of n⁻ semiconductor substrate 1, and has a large width(a large thickness) in the depth direction of the substrate.

In the diode or the IGBT having the above-mentioned structure, in orderto improve the switching characteristics, a method has been known whichgenerates crystal defects in the n⁻ drift layer using electron beamirradiation and controls a carrier lifetime. In addition, in the diodeor the IGBT, in order to reduce switching loss, it is necessary tocontrol carrier concentration at a deep position from the front surface1 b to rear surface 1 a of n⁻ semiconductor substrate 1.

As a method of controlling the carrier concentration in n⁻ semiconductorsubstrate 1 which will be an n⁻ drift layer, a method has been knownwhich performs proton implantation capable of forming a deep range in n⁻semiconductor substrate 1 from rear surface 1 a of n⁻ semiconductorsubstrate 1 at a relatively low acceleration voltage and generates donorlayer 18 a in an n⁻ silicon substrate, which is n⁻ semiconductorsubstrate 1, as shown in FIG. 6. This method performs protonimplantation for a region including oxygen to form n-type FS layer 9 awhich is donor layer 18 a including the crystal defects formed by theproton implantation.

FIG. 7 is a characteristic diagram illustrating a carrier concentrationdistribution on the line X1-X2 of FIG. 6. FIG. 7 illustrates the carrierconcentration distribution of donor layer 18 a which is formed in n⁻semiconductor substrate 1 by the proton implantation. As shown in FIG.7, donor layer 18 a formed by the proton implantation has an impurityconcentration distribution in which impurity concentration has a peakposition at a predetermined depth from rear surface 1 a of n⁻semiconductor substrate 1 and is reduced from the peak position top-type anode region 2 and n⁺ cathode layer 3. In FIG. 7, the verticalaxis is carrier concentration B and the horizontal axis is a depth Cfrom the interface between n⁺ cathode layer 3 and donor layer 18 a(n-type FS layer 9 a).

The proton implantation is used to control a lifetime killer, inaddition to the generation of donors. A method has been known in whichcrystal defects serving as the lifetime killers are generated in thesemiconductor substrate using the proton implantation. The generation ofthe crystal defects in the semiconductor substrate by the protonimplantation makes it possible to control the carrier lifetime of thediode or the IGBT, but has an adverse effect on electricalcharacteristics. For example, the breakdown voltage is reduced or theleakage current is increased by the crystal defects. Therefore, it ispossible to control the amount of crystal defects for generating donorsand the amount of crystal defects which will be the lifetime killers atthe same time.

The following Patent Literature 1 discloses heat treatment conditionsrequired to obtain the desired element characteristics in a method ofgenerating donors using proton implantation. The following PatentLiterature 2 discloses oxygen concentration required to increase thedonor generation rate in the generation of donors by protonimplantation.

Patent Literature 1: United States patent application, Publication No.2006/0286753

Patent Literature 2: Pamphlet of PCT International Publication No.2007/55352

SUMMARY OF THE INVENTION

However, Patent Literature 1 and Patent Literature 2 have the followingproblems. In order to increase the donor generation rate using protonimplantation, it is necessary to appropriately control three elements,that is, hydrogen, oxygen, and crystal defects in the n⁻ drift layer.Patent Literature 1 discloses the degree of recovery of the crystaldefects generated during proton implantation by the heat treatment, butdoes not disclose a method of supplementing the amount of crystaldefects to increase the donor generation rate when the crystal defectsgenerated during proton implantation are insufficient. Patent Literature2 discloses the oxygen concentration required to increase the donorgeneration rate, but does not disclose a method of appropriatelyadjusting the amount of crystal defects to increase the donor generationrate.

The present invention provides a semiconductor device and a method forproducing a semiconductor device capable of increasing a donorgeneration rate using proton implantation and improving electricalcharacteristics, in order to solve the problems of the related artdescribed above.

In order to solve the above-mentioned problems, a method for producing asemiconductor device according to the invention forms a donor layer in asemiconductor substrate using proton implantation and has the followingcharacteristics. First, a crystal defect forming step of generatingcrystal defects before or after the proton implantation is performed.Then, a donor layer forming step of forming the donor layer using theproton implantation and a first heat treatment is performed.

In the method for producing a semiconductor device according to theinvention, the crystal defect forming step may generate the crystaldefects such that a predetermined amount of crystal defects remainduring the first heat treatment.

In the method for producing a semiconductor device according to theinvention, the crystal defect forming step may generate the crystaldefects using electron beam irradiation. In the method for producing thesemiconductor device according to the invention, the crystal defectswhich remain in the semiconductor substrate during the first heattreatment may contribute to the formation of the donor layer.

In the method for producing a semiconductor device according to theinvention, the crystal defect forming step may perform a second heattreatment to adjust the amount of crystal defects after the crystaldefects are formed.

In the method for producing a semiconductor device according to theinvention, the first heat treatment may be performed under conditions ofa temperature of 350° C. to 550° C. and a processing time of 1 to 10hours.

In the method for producing a semiconductor device according to theinvention, the donor layer may be an n-type field stop layer of a diodeor an insulated gate bipolar transistor.

In the method for producing a semiconductor device according to theinvention, a processing temperature of the process performed after thefirst heat treatment may be lower than the temperature of the first heattreatment.

A semiconductor device according to the invention is produced by theabove-mentioned method for producing a semiconductor device.

According to the invention, the electron beam irradiation is performedbefore or after the proton implantation and the amount of crystaldefects is optimally controlled during a heat treatment. Therefore, itis possible to increase the donor generation rate. In addition,according to the invention, when the heat treatment for generatingdonors ends, the crystal defects formed by the electron beam irradiationand the proton implantation are recovered and controlled to anappropriate amount of crystal defects. Therefore, it is possible toimprove electrical characteristics. For example, it is possible toincrease a breakdown voltage and reduce a leakage current.

According to the semiconductor device and the method for producing asemiconductor device of the invention, it is possible to increase thedonor generation rate using proton implantation and improve theelectrical characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing advantages and features of the invention will becomeapparent upon reference to the following detailed description and theaccompanying drawings, of which:

FIGS. 1(a) to 1(h) are cross-sectional views illustrating the sequenceof a production process according to Embodiment 1 of the invention;

FIG. 2 is a flowchart illustrating the flow of the production processshown in FIG. 1;

FIG. 3 is a cross-sectional view illustrating a main portion of asemiconductor device according to Embodiment 1 of the invention which isproduced by the production method illustrated in FIG. 1;

FIG. 4 is a characteristic diagram illustrating electron beamirradiation and a carrier concentration distribution on the line X1-X2of FIG. 3;

FIGS. 5(a) to 5(c) are diagrams illustrating the state of asemiconductor crystal when donors are generated in the semiconductordevice production method according to Embodiment 1;

FIG. 6 is a cross-sectional view illustrating a main portion of a diodeincluding an n-type field stop layer according to the related art;

FIG. 7 is a characteristic diagram illustrating a carrier concentrationdistribution on the line X1-X2 of FIG. 6;

FIGS. 8(a) and 8(b) are characteristic diagrams illustrating adifference in the carrier concentration distribution when electron beamirradiation is performed and when no electron beam irradiation isperformed;

FIG. 9 is a flowchart illustrating the flow of a semiconductor deviceproduction process according to Embodiment 3;

FIG. 10 is a flowchart illustrating the flow of a semiconductor deviceproduction process according to Embodiment 4;

FIGS. 11(a) to 11(h) are cross-sectional views illustrating the sequenceof a production process according to Embodiment 5 of the invention;

FIG. 12 is a characteristic diagram illustrating the relation betweenneutron beam irradiation and a carrier concentration distribution on theline X1-X2 of FIG. 3;

FIG. 13 is a characteristic diagram illustrating the carrier lifetime ofthe semiconductor device according to the invention;

FIGS. 14(a) to 14(c) are characteristic diagrams illustrating therelation between carrier concentration and the average range of protonirradiation in the related art;

FIG. 15 is a characteristic diagram illustrating a threshold voltage atwhich a voltage waveform starts to oscillate;

FIG. 16 is an oscillation waveform during the reverse recovery of ageneral diode;

FIG. 17 is a characteristic diagram illustrating the relation betweenthe range of protons and the acceleration energy of the protons in thesemiconductor device according to the invention;

FIG. 18 is a table illustrating the position conditions of a field stoplayer which a depletion layer reaches initially in the semiconductordevice according to the invention;

FIGS. 19(a) and 19(b) are diagrams illustrating a semiconductor deviceaccording to Embodiment 8; and

FIG. 20 is a characteristic diagram illustrating the reverse recoverywaveform of the semiconductor device according to the invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Hereinafter, a semiconductor device and a method for producing asemiconductor device according to exemplary embodiments of the inventionwill be described in detail with reference to the accompanying drawings.In the specification and the accompanying drawings, in the layers orregions having “n” or “p” appended thereto, an electron or a hole meansa majority carrier, respectively. In addition, symbols “+” and “−” addedto n or p mean that impurity concentration is respectively higher andlower than that of the layer or the region without the symbols. In thedescription of the following embodiments and the accompanying drawings,the same components are denoted by the same reference numerals and thedescription thereof will not be repeated.

Embodiment 1

FIG. 1 is a cross-sectional view illustrating the sequence of aproduction process according to Embodiment 1 of the invention. FIG. 2 isa flowchart illustrating the flow of the production process shown inFIG. 1. Next, a method for producing a semiconductor device according toEmbodiment 1 will be described with reference to FIGS. 1 and 2. Thesemiconductor device manufactured (produced) by the semiconductor deviceproduction method according to Embodiment 1 shown in FIG. 1 is diode 100including n-type field stop (FS) layer 9 illustrated in FIG. 1(h).First, for example, an n-type silicon substrate is prepared as thick n⁻semiconductor substrate 20 which is not thinned (FIG. 1(a)).

Then, as illustrated in the processes (1) and (2) of FIG. 2, p-typeanode region 2 and anode electrode 5 (metal) which is electricallyconnected to p-type anode region 2 are formed on a first main surface(front surface 20 a) of n⁻ semiconductor substrate 20, which will be ann⁻ drift layer, by a general method. In addition, junction edgetermination structure 4 (edge portion) which ensures a breakdown voltageand surrounds p-type anode region 2 and insulating film 8 which coversfront surface 20 a of n⁻ semiconductor substrate 20 are formed in theouter circumference of p-type anode region 2. The junction edgetermination structure 4 includes, for example, a plurality of p-typelayers and a metal electrode which comes into contact with the p-typelayer, which is not illustrated in the drawings.

(1) in FIG. 2 illustrates a process of forming a front surfacestructure, such as p-type anode region 2, the p-type layers of junctionedge termination structure 4, or insulating film 8, as a surface formingprocess. (2) in FIG. 2 illustrates a process of forming surface metal,such as anode electrode 5 or the metal electrode of junction edgetermination structure 4, as a surface electrode forming process. Asillustrated in the process (3) of FIG. 2, for example, a polyimide film,a silicon nitride film (Si₃N₄ film), or a laminated film of a siliconnitride film and a polyimide film, which is a surface protective film(not illustrated), is formed on front surface 20 a of n⁻ semiconductorsubstrate 20. Anode electrode 5 and the surface protective film may beformed after n⁺ cathode layer 3, which will be described below, isformed (FIG. 1(g)). FIG. 1(b) is a cross-sectional view illustratingthis state.

Then, as illustrated in the process of (4) of FIG. 2, electron beamirradiation 11 is performed on front surface 20 a of the n⁻semiconductor substrate 20 to generate crystal defects (for example,point defects) 12 in the n⁻ drift layer. FIG. 1(c) is a cross-sectionalview illustrating this state. In FIG. 1(c), crystal defects 12 arehatched (which holds for FIGS. 1(d) and 1(e)). Electron beam irradiation11 may be performed under the conditions of, for example, accelerationenergy of about 0.5 MeV to 5 MeV and a dose of about 20 kGy to 3000 kGy.After electron beam irradiation 11, pre-heating (second heat treatment)for adjusting the amount of crystal defects 12 may be performed at atemperature of, for example, about 300° C. to 500° C. for 1 to 10 hours.The pre-heating may not be performed when the amount of crystal defects12 formed by electron beam irradiation 11 is proper.

Electron beam irradiation 11 (also including pre-heating when thepre-heating is performed to adjust the amount of crystal defects 12) maybe performed after grinding 21, which will be described below, isperformed for n⁻ semiconductor substrate 1 (FIG. 1(d)) or after protonimplantation 13 (FIG. 1(e)) and before a heat treatment (first heattreatment, FIG. 1(f)) for generating donors. In addition, as a method ofgenerating crystal defects 12, for example, helium irradiation may beperformed, instead of electron beam irradiation 11. However, in somecases, a lifetime killer is introduced by electron beam irradiation 11.

Then, as illustrated in the process (5) of FIG. 2, grinding 21 isperformed on a second main surface (rear surface 20 b) of n⁻semiconductor substrate 20 to reduce the thickness to a predeterminedvalue, for example, of about 100 μm. The thin n⁻ semiconductor substrateafter grinding 21 is denoted by reference numeral 1. FIG. 1(d) is across-sectional view illustrating this state. Then, rear surface 1 a ofthe thin n⁻ semiconductor substrate 1 is cleaned.

Then, as illustrated in the process (6) of FIG. 2, proton implantation13 is performed on rear surface 1 a of n− semiconductor substrate 1 tointroduce hydrogen (H) atoms 14 and crystal defects 15 into n⁻semiconductor substrate 1. FIG. 1(e) is a cross-sectional viewillustrating this state. The conditions of proton implantation 13 maybe, for example, an acceleration energy of 0.4 MeV or more and animplantation dose of about 1×10¹³/cm² to 5×10¹⁴/cm². The amount ofcrystal defects in n⁻ semiconductor substrate 1 is determined by crystaldefects 15 formed by proton implantation 13 and crystal defects 12(including pre-heating when the pre-heating is performed to adjust theamount of crystal defects 12) formed by electron beam irradiation 11.

As such, after proton implantation 13, n⁻ semiconductor substrate 1includes crystal defects 12 formed by electron beam irradiation 11,hydrogen atoms 14 and crystal defects 15 formed by proton implantation13, and oxygen (O) atoms (not illustrated) in the crystal of n⁻semiconductor substrate 1. In FIG. 1(e), hydrogen atoms 14 and crystaldefects 15 are represented by the same mark x (which holds for FIG.11(e)). However, the concentration of the oxygen atoms in n⁻semiconductor substrate 1 is not particularly limited.

Then, as illustrated in the process (7) of FIG. 2, a heat treatment(hereinafter, referred to as a heat treatment for generating donors) isperformed to ionize hydrogen atoms 14 introduced into n⁻ semiconductorsubstrate 1 and to generate donors. The generation of the donors isaccelerated by the heat treatment and donor layer 18 is formed in rearsurface 1 a of n⁻ semiconductor substrate 1. FIG. 1(f) is across-sectional view illustrating this state. Donor layer 18 becomesn-type FS layer 9 of diode 100 and becomes a region with a peak higherthan that of the carrier concentration of n⁻ semiconductor substrate 1.N-type FS layer 9 will be described below. The state of a semiconductorcrystal during the generation of the donors will be described below.

It is important that the temperature of the heat treatment for formingdonor layer 18 is sufficiently low to completely recover crystal defects12 and 15. Specifically, the heat treatment for forming donor layer 18may be performed at a temperature of 350° C. to 550° C. for a processingtime of 1 to 10 hours. The reason is as follows. When the heat treatmentconditions for forming donor layer 18 are greater than theabove-mentioned values (the temperature is higher than 550° C. and theprocessing time is more than 10 hours), the amount of crystal defects 12and 15 is small during the generation of the donors and donor layer 18is insufficiently formed.

When the heat treatment conditions for forming donor layer 18 are lessthan the above-mentioned values (the temperature is lower than 350° C.and the processing time less than 1 hour), the donors are insufficientlygenerated and donor layer 18 is insufficiently formed. In addition, whenthe heat treatment ends, the recovery of crystal defects 12 and 15 isinsufficient and a large amount of crystal defects 12 and 15 remain.Therefore, there are a large amount of lifetime killers in the n⁻ driftlayer, which causes a reduction in breakdown voltage or an increase inleakage current. The preferred conditions of the heat treatment forgenerating donor layer 18 in a good state are, for example, atemperature of about 380° C. to 450° C. and a processing time of about 3to 7 hours.

As such, during the heat treatment for generating donors, it isimportant that crystal defects 12 and 15 formed by electron beamirradiation 11 or proton implantation 13 are not completely recovered,but some of crystal defects 12 and 15 remain. The remaining crystaldefects 12 and 15 contribute to accelerating the generation of donors byprotons. When the heat treatment for generating donors ends, it isimportant to recover crystal defects 12 and 15 such that an appropriateamount of crystal defects 12 and 15 remain in the n⁻ drift layer.

Therefore, the conditions are that crystal defects 12 and 15 formed byproton implantation 13 or electron beam irradiation 11 remain in the n⁻drift layer during the heat treatment for generating donors. Inaddition, after the heat treatment for generating donors ends, it ispreferable that crystal defects 12 and 15 be recovered and anappropriate amount of crystal defects 12 and 15 remain in the n⁻ driftlayer such that desired electrical characteristics are obtained, interms of breakdown voltage, leakage current, on-voltage, and switchingcharacteristics.

Then, as illustrated in the process of FIG. 2(8), after n-type impurityions, such as phosphorus (P) ions, are implanted into rear surface 1 aof n⁻ semiconductor substrate 1, for example, laser annealing isperformed to activate the n-type impurity ions, thereby forming ancathode layer 3. FIG. 1(g) is a cross-sectional view illustrating thisstate. Then, as illustrated in the process of FIG. 2(9), a cathodeelectrode 6 is formed on rear surface 1 a of n⁻ semiconductor substrate1. In this way, diode 100 illustrated in FIG. 1(h) is completed. Aftercathode electrode 6 is formed, a heat treatment may be performed oncathode electrode 6, if necessary.

In the semiconductor device production method according to Embodiment 1,the concentration of crystal defects 12 (FIG. 1(c)) formed by electronbeam irradiation 11 is reduced by the heat treatment for generatingdonors (FIG. 1(f)) and the thermal activation process for formingcathode layer 3 (FIG. 1(g)), but crystal defects 12 remain without beingcompletely recovered. Therefore, the lifetime of minority carriers inthe n⁻ drift layer is equal to or less than 10 μs. In diode 100, inorder to reduce the reverse recovery time, a heat treatment may beperformed under the conditions that crystal defects 12 formed byelectron beam irradiation 11 can remain and the lifetime of the carriersin the n⁻ drift layer may be in the range of about 0.1 μs to 1 μs. Inthis case, for example, after electron beam irradiation 11, a heattreatment may be performed under the conditions of a temperature of 350°C. to 380° C. and a processing time of about 0.5 to 2 hours.

Next, FIG. 3 is a cross-sectional view illustrating a main portion ofdiode 100 produced by the semiconductor device production methodaccording to Embodiment 1. FIG. 3 is a cross-sectional view illustratingthe main portion of the semiconductor device according to Embodiment 1produced by the production method illustrated in FIG. 1. Thesemiconductor device according to Embodiment 1 illustrated in FIG. 3 isdiode 100 with n-type FS layer 9 whose donor generation rate is improvedby electron beam irradiation 11 illustrated in FIG. 1(h).

Diode 100 according to Embodiment 1 illustrated in FIG. 3 differs fromdiode 100 a according to the related art illustrated in FIG. 6 in thatit includes n-type FS layer 9 (donor layer 18) in which crystal defects12 are additionally formed by electron beam irradiation 11, instead ofn-type FS layer 9 a (donor layer 18 a) of diode 100 a according to therelated art illustrated in FIG. 6.

Donor layer 18 which will be n-type FS layer 9 is formed in n⁻semiconductor substrate 1 by electron beam irradiation 11 and protonimplantation 13, as described above, and crystal defects 12 and 15remaining in n⁻ semiconductor substrate 1 contribute to the generationof donors during the heat treatment. In this way, it is possible toobtain diode 100 including n-type FS layer 9 with a higher donorgeneration rate than that in the related art. FIG. 4 illustrates thecarrier concentration distribution of n-type FS layer 9. FIG. 4 is acharacteristic diagram illustrating the relation between electron beamirradiation and a carrier concentration distribution on the line X1-X2of FIG. 3.

As illustrated in FIG. 4, n-type FS layer 9 has an impurityconcentration distribution in which impurity concentration has a peakposition at a predetermined depth from rear surface 1 a of n⁻semiconductor substrate 1 and is reduced from the peak position top-type anode region 2 and n⁺ cathode layer 3. In addition, n-type FSlayer 9 has a carrier concentration distribution in which the donorgeneration rate is higher than that of n-type FS layer 9 a of diode 100a according to the related art by crystal defects 12 formed by electronbeam irradiation 11. In FIG. 4, the vertical axis is carrierconcentration B and the horizontal axis is a depth C from the interfacebetween n⁺ cathode layer 3 and n-type FS layer 9. In FIG. 4, ‘noelectron beam irradiation’ represented by a dotted line indicates diode100 a according to the related art illustrated in FIG. 6 and ‘electronbeam irradiation’ represented by a solid line indicates diode 100according to Embodiment 1 of the invention.

As represented by the solid line in FIG. 4, a method of adding crystaldefects 12 using electron beam irradiation 11 can improve the donorgeneration rate. For example, the donor generation rate in the relatedart is 1%, but the donor generation rate in this embodiment can increaseabout 3%. That is, the carrier concentration is about three times higherthan that in the related art. As a result, for example, when a donorpeak concentration of 1×10¹⁵/cm³ is needed, the amount of hydrogenimplanted (the amount of proton implanted) can be reduced to aboutone-third of the amount of hydrogen implanted in the related art. Inaddition, the amount of oxygen in the n⁻ semiconductor substrate can beless than that in the related art. As such, since the amount of protonimplanted can be reduced, the amount of crystal defects can be reducedand mobility can be improved. As a result, it is possible to reduce theon-voltage, increase the breakdown voltage, and reduce the leakagecurrent.

In the semiconductor device production method according to Embodiment 1,in some cases, after anode electrode 5 of the semiconductor device(diode 100) is formed (after the processes of FIG. 1(g) or 1(h)), acopper or nickel gold plating layer for soldering is formed on anodeelectrode 5. In this case, the temperature when the plating layer isformed on anode electrode 5 needs to be lower than the temperature ofthe heat treatment for generating the donors. When a lead frame, whichis an external lead terminal, is soldered to the plating layer, asoldering temperature needs to be lower than the temperature of the heattreatment for generating the donors. When the surface protective film isformed after the heat treatment for generating donors, the temperatureof the heat treatment for forming the surface protective needs to belower than the temperature of the heat treatment for generating donors.

That is, the formation temperature of each portion formed after the heattreatment for generating the donors needs to be lower than thetemperature of the heat treatment for generating the donors. The reasonis that, when the formation temperature of each portion formed after theheat treatment for generating the donors is higher than the temperatureof the heat treatment for generating the donors, the donors generated bythe heat treatment for generating the donors are relaxed and return to astate close to a normal crystal state and the diffusion concentrationdistribution of n-type FS layer 9 deteriorates.

Next, the state of the semiconductor crystal during the generation ofthe donors will be described using a case in which n⁻ semiconductorsubstrate 1 is, for example, a silicon (Si) substrate as an example.FIG. 5 is a diagram illustrating the state of the semiconductor crystalduring the generation of the donors in the semiconductor deviceproduction method according to Embodiment 1. For example, the bondbetween silicon atoms in a silicon crystal is cleaved and the siliconatoms in the silicon crystal are emitted by electron beam irradiation 11and proton implantation 13. As a result, crystal defects are generated.In addition, the hydrogen ions (H⁺) introduced by proton implantation 13enter the silicon crystal, capture free electrons in the siliconcrystal, and become interstitial hydrogen (H⁺) atoms (FIG. 5(a)).

The crystal defects generated by electron beam irradiation 11 and protonimplantation 13 have silicon atom dangling bonds (FIG. 5(b)). In aportion in which the bond between the silicon atoms is cleaved, the heattreatment causes the silicon atom to be substituted with the hydrogenatom through the dangling bond. In addition, the emitted silicon atom issubstituted with the hydrogen atom. The substituted hydrogen atom emitsan electron, has positive charge, is ionized into a hydrogen ion (H⁺),and becomes a donor with an extra electron like a group 15 element, suchas phosphorus (P) (FIG. 5(c)). Here, the involvement of oxygen will notbe described.

As such, a heat treatment is performed, with three elements, that is, acrystal defect, hydrogen, and oxygen included in the silicon substrate,to generate donors. It is important that three elements, that is, acrystal defect, hydrogen, and oxygen are appropriately present in orderto improve the donor generation rate. In the invention, in particular,when the amount of crystal defects formed by proton implantation isinsufficient, it is possible to increase the amount of crystal defectsusing electron beam irradiation 11 before the heat treatment forgenerating donors. Therefore, it is possible to ensure the amount ofcrystal defects required to generate donors and improve the donorgeneration rate. For this reason, the invention is useful.

As described above, according to Embodiment 1, electron beam irradiationis performed to generate crystal defects in the n⁻ semiconductorsubstrate. In addition, the crystal defects in the n⁻ semiconductorsubstrate are not completely recovered, but some of the crystal defectsremain during the heat treatment for generating donors which isperformed after proton implantation. Therefore, it is possible toimprove the donor generation rate, as compared to the related art.According to Embodiment 1, when the heat treatment for generating donorsends, the crystal defects formed by electron beam irradiation and protonimplantation are recovered and are controlled such that the amount ofcrystal defects is appropriate. Therefore, it is possible to improveelectrical characteristics. That is, for example, it is possible toimprove the breakdown voltage and reduce the leakage current.

Embodiment 2

FIG. 8 is a characteristic diagram illustrating a difference in carrierconcentration distribution when electron beam irradiation is performedand when no electron beam irradiation is performed. FIG. 8(a) is adiagram illustrating the comparison between an example according to theinvention in which electron beam irradiation is performed (hereinafter,referred to as an example) and an example according to the related artin which no electron beam irradiation is performed (hereinafter,referred to as Conventional example 1). FIG. 8(b) is a diagramillustrating the comparison between an example in which no electron beamirradiation is performed and Rp2 is equal to or more than 0.5Rp1(hereinafter, referred to as Conventional example 2) and an example inwhich no electron beam irradiation is performed and Rp2 is less than0.5Rp1 (Conventional example 1). Here, Rp1 and Rp2 are protonimplantation ranges and are average ranges from rear surface 1 a of n⁻semiconductor substrate 1. The average range means the depth of a peakconcentration position of the impurity concentration distribution ofn-type FS layer 9, which is represented by a Gaussian distribution, fromthe rear surface of the substrate. Specifically, the average range is adepth from the rear surface of the substrate to the proton peakposition.

A semiconductor device production method according to Embodiment 2differs from the semiconductor device production method according toEmbodiment 1 in that proton implantation is performed a plurality oftimes (hereinafter, referred to as in a plurality of stages).Specifically, first proton implantation (first stage) is performed inwhich the deepest position from rear surface 1 a of n⁻ semiconductorsubstrate 1 is a range Rp1 and then second proton implantation (secondstage) is performed in which a range Rp2 is less than half of the rangeRp1 of the first proton implantation. In this case, the range Rp2 of thesecond proton implantation is less than 0.5Rp1 in both this example andConventional example 1 (Rp2<0.5Rp1). On the other hand, in Conventionalexample 2, the second proton implantation is performed in a range Rp2 bequal to or more than 0.5Rp1 (Rp2 b≧0.5Rp1). In this example andConventional examples 1 and 2, third proton implantation (third stage)in which a range Rp3 is about 5 μm is performed. In this example andConventional examples 1 and 2, electron beam irradiation was performedin the same sequence as that in Embodiment 1 (FIG. 2(4)).

First, as illustrated in FIG. 8(b), among a plurality of (three) protonimplantation operations, in Conventional example 2 in which the rangeRp2 b in the second stage is equal to or more than 0.5Rp1 with respectto the first stage with the deepest range, carrier concentration is notreduced in a region (hereinafter, referred to as a region A) between thefirst stage (the deepest range Rp1) and the second stage (Rp2 b).However, in Conventional example 1 in which the range Rp2 in the secondstage is less (shallower) than 0.5Rp1, the carrier concentration isgreatly reduced in the region A. The reason is as follows. Whenspreading resistance is converted into resistivity (carrierconcentration), a spread-resistance profiling (SR) method uses thetheoretical value (in the case of an n type, the electron mobility isabout 1360 cm²/(Vs)) of silicon carrier mobility. That is, sinceimplantation damage (various crystal defects occur in silicon anddisorder occurs) is introduced by the proton implantation, the actualmobility is greatly reduced. The reduction in the mobility occurs inConventional example 1 illustrated in FIG. 8 and apparently, carrierconcentration is reduced. As such, it is presumed that, when the rangeRp2 in the second stage is less than 0.5Rp1, the reduction in themobility in the region A is not recovered by the implanted hydrogen andcarrier concentration is greatly reduced.

As in Example 1, electron beam irradiation 11 is performed to introducea large amount of point defects (vacancies and divacancies) before thefirst to third proton implantation operations. The result illustrated inFIG. 8(a) illustrates that, even when the range Rp2 in the second stageis less than 0.5Rp1, carrier concentration is sufficiently recovered.

As described above, according to Embodiment 2, it is possible to obtainthe same effect as that in Embodiment 1.

Embodiment 3

FIG. 9 is a flowchart illustrating the flow of a semiconductor deviceproduction process according to Embodiment 3. The semiconductor deviceproduction method according to Embodiment 3 is a modification of thesemiconductor device production method according to Embodiment 1 inwhich the order of processes before and after proton implantation ischanged. The semiconductor device production method according toEmbodiment 3 differs from the semiconductor device production methodaccording to Embodiment 1 in that, after a rear surface of an n⁻semiconductor substrate is ground and before proton implantation, ionimplantation for forming a cathode layer is performed and the cathodelayer is activated by laser annealing. The other processes in thesemiconductor device production method according to Embodiment 3 are thesame as those in the semiconductor device production method according toEmbodiment 1.

Specifically, first, similarly to Embodiment 1, a surface formingprocess (FIG. 9(1)) to a rear surface grinding process (FIG. 9(5)) areperformed. Then, ion implantation for forming the cathode layer isperformed (FIG. 9(6)). The ion implantation for forming the cathodelayer may be performed under the conditions of, for example, aphosphorus dose of 1×10¹⁵/cm² and an acceleration energy of 50 keV.Then, the cathode layer is activated by laser annealing (FIG. 9(7)). Theion implantation for forming the cathode layer and the activation of thecathode layer by laser annealing may be performed by the same method asthat in Embodiment 1 except that they are performed at a different timefrom those in Embodiment 1. Then, similarly to Embodiment 1, protonimplantation is performed on a rear surface (ground surface) of the n⁻semiconductor substrate and a heat treatment for generating donors isperformed. Then, the subsequent processes are performed (FIGS. 9(8) to9(10)) to complete diode 100 illustrated in FIG. 3.

As described above, according to Embodiment 3, it is possible to obtainthe same effect as that in Embodiment 1.

Embodiment 4

FIG. 10 is a flowchart illustrating the flow of a semiconductor deviceproduction process according to Embodiment 4. A semiconductor deviceproduction method according to Embodiment 4 differs from thesemiconductor device production method according to Embodiment 3 in thatthe activation of a cathode layer by laser annealing is performedtogether with a heat treatment for generating donors which is performedafter proton implantation. That is, in Embodiment 4, laser annealing foractivating the cathode layer is not performed immediately after ionimplantation (FIG. 10(6)) for forming a cathode layer, but proton donorsand the cathode layer are activated at the same time by a heat treatment(FIG. 10(8)) after proton implantation (FIG. 10(7)). The other processesin the semiconductor device production method according to Embodiment 4are the same as those in the semiconductor device production methodaccording to Embodiment 3.

As described above, according to Embodiment 4, it is possible to obtainthe same effect as that in Embodiment 1. In addition, according toEmbodiment 4, it is possible to omit the laser annealing process, reducean annealing device introduction cost, and improve throughput, inaddition to obtaining the same effect as that in Embodiment 1.

Embodiment 5

In Embodiment 1, electron beam irradiation is performed to introduce thepoint defects in the depth direction of the substrate. As an FZsubstrate doping method, a method has been known which performs nucleartransmutation (Si→phosphorus) using neutron beams to dope a drift layeruniformly at a low concentration. A semiconductor device productionmethod according to Embodiment 5 differs from the semiconductor deviceproduction method according to Embodiment 1 in that crystal defects 42are generated by neutron beam irradiation. Crystal defects 42 generatedby the neutron beam irradiation are used to accelerate the change ofprotons to donors.

FIG. 11 is a cross-sectional view illustrating the sequence of aproduction process according to Embodiment 5 of the invention. First, aneutron beam (not illustrated) is irradiated to an ingot produced by anon-doped floating zone (FZ) method to form FZ wafer (n⁻ semiconductorsubstrate) 40 with a resistivity of, for example, 50 Ωcm (FIG. 11(a)).Then, a front surface structure (for example, a p anode layer) orsurface metal (for example, an anode electrode 5) is formed on frontsurface 40 a of FZ wafer 40 at a temperature lower than 1000° C. (FIG.11(b)). The reason why the front surface structure or the surface metalis formed at a temperature lower than 1000° C. is that crystal defects42 generated by the neutron beam irradiation remain in FZ wafer 40. InFIG. 11(a), crystal defects 42 generated by the neutron beam irradiationare hatched (which holds for FIGS. 11(b) to 11(e)). Then, a passivationfilm (not illustrated) made of, for example, polyimide is formed onfront surface 40 a of FZ wafer 40 and the surface forming process ends(FIG. 11(c)).

Then, similarly to Embodiment 1, grinding 21 is performed on rearsurface 40 b of FZ wafer 40 to reduce the thickness of FZ wafer 40 (FIG.11(d)). Hereinafter, the FZ wafer thinned by grinding 21 is denoted byreference numeral 41. Then, proton implantation is performed on theground rear surface 41 a of the FZ wafer 41 once or a plurality of times(FIG. 11(e)). When the proton implantation is performed once, it is thesame as proton implantation 13 in Embodiment 1. When the protonimplantation is performed a plurality of times, it is the same as thatthe proton implantation in Embodiment 2. FIG. 11(e) illustrates a statein which hydrogen atoms 14 and crystal defects 15 are introduced into FZwafer 41 by one proton implantation operation 13.

Then, a heat treatment for generating donors is performed to change(activate) hydrogen atoms 14 into donors using protons (FIG. 11(f)). Inthis way, the same donor layer 18 as that in Embodiment 1 is formed.Then, similarly to Embodiment 1, n⁺ cathode layer 3 is formed on rearsurface 41 a of FZ wafer 41 (FIG. 11(g)) and a rear surface electrode,which will be cathode electrode 6, is formed by, for example,sputtering. In this way, diode 100 illustrated in FIG. 3 is completed(FIG. 11(h)). In FIG. 3, FZ wafer 41 is denoted by reference numeral 1.

FIG. 12 is a characteristic diagram illustrating the relation betweenneutron beam irradiation and a carrier concentration distribution on theline X1-X2 of FIG. 3. FIG. 12 is a diagram illustrating the comparisonbetween carrier concentrations after the completion of processes when aneutron beam is irradiated (diode 100 according to Embodiment 5 of theinvention) and when no neutron beam is irradiated (diode 100 a accordingto the related art illustrated in FIG. 6). The carrier concentration ofn-type FS layer 9 a when a neutron beam is irradiated is less than thatwhen an electron beam is irradiated (see FIG. 4). However, thegeneration of donors is accelerated by crystal defects (point defects)42 generated by the neutron beam irradiation and the carrierconcentration is higher than that in diode 100 a according to therelated art.

As described above, according to Embodiment 5, it is possible to obtainthe same effect as that in Embodiment 1.

Embodiment 6

In Embodiment 6, the preferred proton peak position in the first protonirradiation operation among a plurality of proton irradiation operationin the semiconductor device production method according to Embodiment 2,particularly, the reason why the range Rp1 in the first stage ispreferably equal to or more than a depth of 15 μm from the rear surfaceof the substrate will be described below.

FIG. 16 illustrates an oscillation waveform during the reverse recoveryof a general diode. When an anode current is equal to or less thanone-tenth of the rated current, in some cases, the amount of carriersaccumulated is small and oscillation occurs before the reverse recoveryends. The anode current is fixed to a certain value and the diode isreversely recovered by a different power supply voltage V_(CC). In thiscase, when the power supply voltage V_(CC) is greater than apredetermined value, it is greater than the peak value of a generalovershoot voltage in the voltage waveform between the cathode and theanode and an additional overshoot occurs. The additional overshoot(voltage) serves as a trigger and the subsequent waveform oscillates.When the power supply voltage V_(CC) is greater than the predeterminedvalue, the additional overshoot voltage further increases and thesubsequent oscillation amplitude also increases. As such, a thresholdvoltage at which the voltage waveform starts to oscillate is referred toas an oscillation start threshold value V_(RRO). When the oscillationstart threshold value V_(RRO) increases, the diode does not oscillateduring reverse recovery, which is preferable.

The oscillation start threshold value V_(RRO) depends on the firstproton peak position that a depletion layer (strictly, a space chargeregion including holes) which is spread from the pn junction between ap-type anode layer and an n⁻ drift layer of the diode to the n⁻ driftlayer reaches initially, among a plurality of proton peak positions. Thereason is as follows. When the depletion layer is spread from the p-typeanode layer in the front surface to the n⁻ drift layer surface duringreverse recovery, the end of the depletion layer reaches the first FSlayer (field stop layer) and the spreading of the depletion layer issuppressed. The sweep of the accumulated carriers is weakened. As aresult, the depletion of the carriers is suppressed and oscillation issuppressed.

During reverse recovery, the depletion layer is spread from the pnjunction between the p anode layer and the n⁻ drift layer toward thecathode electrode in the depth direction. Therefore, the peak positionof the FS layer which the end of the depletion layer reaches initiallyis the FS layer which is closest to the pn junction. Here, the thicknessof the n⁻ semiconductor substrate (the thickness of a portion interposedbetween the anode electrode and the cathode electrode) is W0 and thedepth of the peak position of the FS layer which the end of thedepletion layer reaches first from the interface between the cathodeelectrode and the rear surface of the n⁻ semiconductor substrate(hereinafter, referred to as a distance from the rear surface) is X. Adistance index L is introduced. The distance index L is represented bythe following Expression (1).

$\begin{matrix}{L = \sqrt{\frac{ɛ_{S}V_{rate}}{\left( {\frac{J_{F}}{{qv}_{sat}} + N_{d}} \right)}}} & {{Expression}\mspace{14mu} (1)}\end{matrix}$

The distance index L represented by the above-mentioned Expression (1)is an index indicating the distance of the end (depletion layer end) ofthe depletion layer (exactly, a space charge region), which is spreadfrom the pn junction to the n⁻ drift layer, from the pn junction duringreverse recovery when a voltage between the cathode and the anode isV_(CE) and a power supply voltage is V_(CC). In a fraction in the squareroot, a denominator indicates the space charge density of the spacecharge region (simply, the depletion layer) during reverse recovery. Theknown Poisson equation is represented by divE=ρ/ε, in which E iselectric field intensity, ρ is space charge density, and ρ=q(p−n+N_(d)−N_(a)) is established. Here, q is an elementary charge, p ishole concentration, n is electron concentration, N_(d) is donorconcentration, N_(a) is acceptor concentration, and c is thepermittivity of a semiconductor.

The space charge density ρ is described by the hole concentration ρpassing through the space charge region (depletion layer) during reverserecovery and the average donor concentration N_(d) of the n⁻ driftlayer. The electron concentration is lower than these concentrations soas to be negligible and there is no acceptor. Therefore, ρ≈q(p+N_(d)) isestablished. In this case, the hole concentration p is determined by abreaking current of the diode. In particular, on an assumption in whichthe current is flowing, the rated current density of the element isrepresented as p=JF/(qv_(sat)). In addition, JF is the rated currentdensity of the element and v_(sat) is a saturated velocity in which thespeed of the carrier is saturated with predetermined electric fieldintensity.

The Poisson equation is integrated with respect to the distance x twotimes and the voltage V satisfies E=−gradV (the relation between theknown electric field E and the voltage V). Therefore, under appropriateboundary conditions, V=(½)(ρ/ε)x² is established. The length x of thespace charge region obtained when the voltage V is half of the ratedvoltage BV is the distance index L. This is because an operation voltage(power supply voltage), which is the voltage V, is about half of therated voltage in the actual device, such as an inverter. When the dopingconcentration of the FS layer is higher than that of the n⁻ drift layer,the FS layer prevents the expansion of the space charge region which isspread during reverse recovery. In a case in which the anode current ofthe diode starts to be reduced from the breaking current due to theturn-off of a MOS gate, when the peak position of the FS layer which thedepletion layer reaches initially is within the length range of thespace charge region, it is possible to suppress the expansion of thespace charge region, with the accumulated carriers remaining in the n⁻drift layer. Therefore, the sweep of the remaining carriers issuppressed.

In the actual reverse recovery operation, for example, when a motor isdriven by a known PWM inverter with an IGBT module, the power supplyvoltage or the breaking current is not fixed, but is variable.Therefore, in this case, the preferred peak position of the FS layerwhich the depletion layer reaches initially needs to have a given width.According to the results of the inventors' research, the distance X ofthe peak position of the FS layer which the depletion layer reachesinitially from the rear surface is as illustrated in FIG. 18. FIG. 18 isa table illustrating the position conditions of the field stop layerwhich the depletion layer reaches initially in the semiconductor deviceaccording to the invention. FIG. 18 illustrates the distance X of thepeak position of the FS layer which the end of the depletion layerreaches initially from the rear surface at a rated voltage of 600 V to6500 V. Here, X=W0−γL is established and γ is a coefficient. FIG. 18illustrates the distance X when γ is changed from 0.7 to 1.6.

As illustrated in FIG. 18, at each rated voltage, the element (diode) issafely designed so as to have a breakdown voltage which is about 10%higher than the rated voltage. As illustrated in FIG. 18, the totalthickness of the n⁻ semiconductor substrate (the thickness during afinishing process after the n⁻ semiconductor substrate is thinned by,for example, grinding) and the average resistivity of the n⁻ drift layerare set such that an on-voltage or reverse recovery loss is sufficientlyreduced. The term ‘average’ means the average concentration andresistivity of the entire n⁻ drift layer including the FS layer. Therated current density has the typical value illustrated in FIG. 18 dueto the rated voltage. The rated current density is set such that energydensity which is determined by the product of the rated voltage and therated current density has a substantially constant value and has nearlythe value illustrated in FIG. 18. When the distance index L iscalculated using these values according the above-mentioned Expression(1), the value illustrated in FIG. 18 is obtained. The distance X of thepeak position of the FS layer which the end of the depletion layerreaches initially from the rear surface is obtained by subtracting theproduct of the distance index L and γ, which is in the range of 0.7 to1.6, from the thickness W0 of the n⁻ semiconductor substrate.

The distance X of the peak position of the FS layer which the end of thedepletion layer reaches initially from the rear surface, at whichreverse recovery oscillation is sufficiently suppressed, is as followswith respect to the distance index L and the thickness W0 of the n⁻semiconductor substrate. FIG. 15 is a characteristic diagramillustrating a threshold voltage at which the voltage waveform starts tooscillate. FIG. 15 is a graph illustrating the dependence of V_(RRO) onγ at some typical rated voltages V_(rate) (600 V, 1200 V, and 3300 V).In the graph, the vertical axis indicates a value obtained bystandardizing V_(RRO) with the rated voltage V_(rate). As can be seenfrom the graph, it is possible to rapidly increase V_(RRO) at threerated voltages when γ is equal to or less than 1.4. When γ is in therange of 0.8 to 1.3, it is possible to sufficiently increase V_(RRO) atany rated voltage. More preferably, when γ is in the range of 0.9 to1.2, it is possible to maximize V_(RRO).

The important point in FIG. 15 is that the range of γ capable ofsufficiently increasing V_(RRO) is substantially the same (0.8 to 1.3)at any rated voltage. The reason is that it is most effective to set therange of the distance X of the peak position of the FS layer which thedepletion layer reaches initially from the rear surface to be centeredon W0−L (γ=1). That is, the above is caused by the nearly constant valueof the product of the rated voltage and the rated current density.Therefore, when the distance X of the peak position of the FS layerwhich the end of the depletion layer reaches initially from the rearsurface is set in the above-mentioned range, the accumulated carrierscan sufficiently remain in the diode during reverse recovery and it ispossible to suppress the oscillation phenomenon during reverse recovery.Therefore, for the distance X of the peak position of the FS layer whichthe end of the depletion layer reaches initially from the rear surface,it is preferable that the coefficient γ of the distance index L be inthe above-mentioned range at any rated voltage. In this way, it ispossible to effectively suppress the oscillation phenomenon duringreverse recovery. In addition, the depth of the first stage from therear surface is γ=1, that is, the range Rp1 of the first stage from therear surface of the substrate is equal to or more than 15 μm in order tomaximize the effect of suppressing oscillation.

As such, in order to obtain good switching characteristics, it isnecessary to form the FS layer in a region which is at a depth of atleast 15 μm from the rear surface of the semiconductor substrate. Theinventors found that, when the average range of proton irradiation wasset to 15 μm or more in order to form the FS layer in the region whichis at a depth of 15 μm or more from the rear surface of thesemiconductor substrate, the region in which protons reached a depth of15 μm from the rear surface of the semiconductor substrate was a regionin which carrier concentration by the SR method was significantly lowerthan the doping concentration of the semiconductor substrate, that is, adisorder region. This will be described with reference to FIG. 14.

FIG. 14 is a characteristic diagram illustrating the relation betweencarrier concentration and the average range of proton irradiation in therelated art. FIG. 14 illustrates the carrier concentration of thesilicon substrate measured by the SR method after protons are irradiatedto the silicon substrate and then a heat treatment is performed at 350°C. FIG. 14(a) illustrates a case in which the average range of theproton irradiation is 50 μm, FIG. 14(b) illustrates a case in which theaverage range of the proton irradiation is 20 μm, and FIG. 14(c)illustrates a case in which the average range of the proton irradiationis 10 μm. In FIGS. 14(a) to 14(c), the horizontal axis is the distance(depth) of the proton from an incident surface. In FIG. 14(c), when theaverage range of the proton irradiation is 10 μm, particularly, areduction in carrier concentration does not appear in a proton passageregion. In FIG. 8(b), when the average range of the proton irradiationis 20 μm, carrier concentration is lower than substrate concentrationand a reduction in the carrier concentration appears. That is, disorderremains in the region. As can be seen from FIG. 14(a), when the averagerange of the proton irradiation is 50 μm, a reduction in the carrierconcentration of the passage region is remarkable and a large amount ofdisorder remains. As such, when there is a disorder region in thesemiconductor substrate, a leakage current or conduction loss increases,as described above. Therefore, it is necessary to remove disorder.

As described above, even when the range Rp (Rp1) of the first stage issufficiently deep, carrier mobility is sufficiently recovered and it isimportant that carrier concentration is equal to or more than substrateconcentration. The invention can solve the above-mentioned problems byperforming proton irradiation a plurality of times.

The acceleration energy of protons may be determined from the followingcharacteristics graph illustrated in FIG. 17 in order to form the FSlayer with the peak position which the depletion layer initially reachesand which is at the distance X from the rear surface using protonirradiation such that the above-mentioned range of γ is satisfied inpractice.

The results of the inventors' research proved that, for the range Rp(the peak position of the FS layer) of protons and the accelerationenergy E of protons, when the logarithm log(Rp) of the range Rp of theprotons was x and the logarithm log(E) of the acceleration energy E ofthe protons was y, x and y satisfied the following relation representedby Expression (2).

y=−0.0047x ⁴+0.0528x ³−0.2211x ²+0.9923x+5.0474  Expression (2)

FIG. 17 is a characteristics graph indicating the above-mentionedExpression (2). FIG. 17 is a characteristic diagram illustrating therelation between the range of the protons and the acceleration energy ofthe protons in the semiconductor device according to the invention. FIG.17 illustrates the acceleration energy of the protons for obtaining thedesired range of the protons. In FIG. 17, the horizontal axis is thelogarithm log(Rp) of the range Rp of the protons and indicates the rangeRp (μm) corresponding to the parentheses below the axis value oflog(Rp). In addition, the vertical axis is the logarithm log(E) of theacceleration energy E of the protons and indicates the accelerationenergy E of the protons corresponding to the parentheses below the axisvalue of log(E). The above-mentioned Expression (2) is obtained byfitting the value of the logarithm log(Rp) of the range Rp of theprotons and the value of the logarithm log(E) of the acceleration energywith a four-order polynomial of x (=log(Rp)).

The following relation may be considered between the actual accelerationenergy E′ and an average range Rp′ (proton peak position) which isactually obtained by the spreading resistance (SR) method when theabove-mentioned fitting equation is used to calculate the accelerationenergy E (hereinafter, referred to as the calculated value E) of protonirradiation from the desired average range Rp of protons and protons areimplanted into the silicon substrate with the calculated value E of theacceleration energy. When the actual acceleration energy E′ is in therange of about ±10% of the calculated value E of the accelerationenergy, the actual average range Rp′ is within the range of about ±10%of the desired average range Rp and is in a measurement error range.Therefore, the influence of a variation in the actual average range Rp′from the desired average range Rp on the electrical characteristics ofan IGBT is so small to be negligible. When the actual accelerationenergy E′ is in the range of ±5% of the calculated value E, it ispossible to determined that the actual average range Rp′ issubstantially equal to the set average range Rp. Alternatively, when theactual average range Rp′ is in the range of about ±10% of the averagerange Rp obtained by applying the actual acceleration energy E′ to theabove-mentioned Expression (2), no problem occurs. In the actualaccelerator, since the acceleration energy E and the average range Rpare both in the above-mentioned range (±10%), it is considered that theactual acceleration energy E′ and the actual average range Rp′ followthe above-mentioned fitting equation represented by the desired averagerange Rp and the calculated value E and no problem occurs. In addition,a variation or error range may be equal to or less than ±10% of theaverage range Rp. When the variation or error range is within the rangeof ±5%, it can be considered that the actual acceleration energy E′ andthe actual average range Rp′ follow the above-mentioned Expression (2).

The above-mentioned Expression (2) is used to calculate the accelerationenergy E of the protons required to obtain the desired range Rp of theprotons. The acceleration energy E of each proton for forming the FSlayer is also calculated by the above-mentioned Expression (2) and iswell matched with the value of a sample which is measured by protonirradiation with the acceleration energy E′ and the known spreadingresistance measurement method (SR method). Therefore, when theabove-mentioned Expression (2) is used, it is possible to predict therequired acceleration energy E of protons with high accuracy, on thebasis of the range Rp of the protons.

As described above, according to Embodiment 6, it is possible to obtainthe same effect as that in Embodiment 1.

Embodiment 7

FIG. 20 is a characteristic diagram illustrating the reverse recoverywaveform of a semiconductor device according to the invention. FIG. 20illustrates the reverse recovery waveform of the semiconductor deviceaccording to Embodiment 1 of the invention and a reverse recoverywaveform according to a comparative example when proton implantation isnot performed, but only electron beam irradiation is performed. A ratedvoltage was 1200 V. A silicon substrate produced by the FZ method wasused, the doping concentration (average concentration) N_(d) of thesilicon substrate and the thickness W0 of the silicon substrate aftergrinding had values at a rated voltage of 1200 V illustrated in FIG. 18.In the invention, γ (corresponding to the range Rp1 in the first stage)is 1. In the invention, electron beam irradiation conditions were a doseof 300 kGy and acceleration energy of 5 MeV. In the comparative example,electron beam irradiation conditions were a dose of 60 kGy. In both theinvention and the comparative example, a forward voltage drop at therated current density (a 1200-V field in FIG. 18) was 1.8 V. The test isperformed under the conditions that a power supply voltage V_(CC) is 800V and the initial normal anode current is the rated current (currentdensity x an active area (about 1 cm²)). In a chopper circuit, flowinginductance with respect to a diode, a driving IGBT (similarly, 1200 V),and an intermediate capacitor is 200 nH.

As can be seen from FIG. 20, in the invention, a reverse recovery peakcurrent is smaller than that in the comparative example and an overshootvoltage higher than the power supply voltage V_(CC) is about 200 V lowerthan that in the comparative example. That is, the reverse recoverywaveform according to the invention is a so-called soft recoverywaveform. Therefore, even when lifetime control is performed by electronbeam irradiation in which high-speed and hard recovery is likely tooccur, the reverse recovery waveform according to the invention can be avery soft waveform, which is a peculiar effect that is not obtained inthe related art.

The effect and operation (reason) of the invention will be describedwith reference to FIG. 13. FIG. 13 is a characteristic diagramillustrating the carrier lifetime of the semiconductor device accordingto the invention. FIG. 13 illustrates net doping concentration, pointdefect concentration, and carrier lifetime in the depth direction ofanode electrode 5 in the diode which is produced according to Embodiment2. In FIG. 13, reference numeral 9 (reference numerals 9 a to 9 c)denotes an n-type FS layer. It is presumed that the effect of theinvention is obtained because the dangling bond is terminated byhydrogen atoms which are introduced by the implantation of protons intothe rear surface of the substrate in the point defects (vacancies anddivacancies) introduced by electron beam irradiation. The crystal defectwhich accelerates the generation and extinction of carriers is mainly apoint defect and is energy center having a vacancy (V) and a divacancy(VV) as main components. In addition, the dangling bond is formed in thepoint defect (see FIG. 5). Protons are implanted into the portion inwhich the dangling bond is formed from the rear surface of the substrateand a heat treatment for generating donors is performed. Then, thecrystal defects are relaxed and the portion returns to a state close tothe normal crystal state. In this case, the dangling bond is terminatedby a peripheral hydrogen atom. Therefore, the center including thevacancy (V) and the divacancy (VV) as main components disappears. On theother hand, as in the invention, when a donor (hydrogen induced donor)caused by the hydrogen atom is generated, a VOH defect of a vacancy (V),oxygen (O), and hydrogen (H) is a main component in the hydrogen induceddonor. Therefore, the VOH defect is also formed only by the terminationof the dangling bond by the hydrogen atom. It is presumed that the VOHdefect accelerates the generation of a VOH donor while reducing thedensity of the vacancies (V) and the divacancies (VV) causing a leakagecurrent or carrier recombination.

As illustrated in the middle part of FIG. 13, for the density of thepoint defects, a sufficiently amount of point defects generated byelectron beam irradiation remain from the anode to n-type FS layer 9 anduniform lifetime distribution is formed by the above-mentionedphenomenon. In this case, the lifetime is, for example, equal to or morethan 0.1 μs and equal to or less than 3 μs. In a portion from n-type FSlayer 9 to the cathode on the rear surface of the substrate, hydrogenconcentration is increased by proton implantation at a depth of about 70μm or more (that is, closer to the cathode) from the front surface ofthe substrate. Since the dangling bond is terminated by the hydrogenatom, point defect concentration is reduced. The carrier lifetime on therear surface side of the substrate is longer than the carrier lifetime,0.1 μs to 3 μs on the front surface side of the substrate and is, forexample, about 10 μs. The carrier lifetime on the rear surface side ofthe substrate is equal to or sufficiently close to the carrier lifetime(10 μs or more) when electron beam irradiation is not performed.Therefore, the concentration distribution of the minority carriers (notillustrated) (in this case, holes) is sufficiently low on the anode sideand is sufficiently high on the cathode. As a result, an ideal carrierconcentration distribution is obtained by the soft recoverycharacteristics of the diode.

As described above, according to Embodiment 7, it is possible to obtainthe same effect as that in Embodiment 1.

Embodiment 8

FIG. 19 is a diagram illustrating a semiconductor device according toEmbodiment 8. The semiconductor device according to Embodiment 8 is anexample in which the structure of the semiconductor device according toEmbodiment 1 is applied to an IGBT. FIG. 19(a) illustrates thecross-sectional structure of the IGBT and FIG. 19(b) illustrates a netdoping concentration distribution on the line A-A′ of FIG. 19(a). In amethod for producing the semiconductor device according to Embodiment 8,the element structure of the IGBT may be formed, instead of the elementstructure of the diode in the semiconductor device production methodsaccording to Embodiments 1 to 5.

As illustrated in FIG. 19, in the IGBT, it is possible to obtain theeffect of accelerating the change of hydrogen (related defect) into adonor by controlling point defect concentration using electron beamirradiation or neutron beam irradiation, similarly the diode accordingto Embodiment 1. In the IGBT, the carrier lifetime is not activelyreduced, unlike the diode. Therefore, the average of the carrierlifetime may be equal to or less than 10 μs by a heat treatment which isperformed after proton implantation. In this case, the temperature ofthe heat treatment is preferably equal to or higher than, for example,380° C. and more preferably, equal to or higher than 400° C. and equalto or lower than 450° C.

In FIG. 19, reference numeral 9 (reference numerals 9 a to 9 c) denotesan n-type FS layer. Reference numeral 31 denotes an emitter electrode,reference numeral 32 denotes a collector electrode, reference numeral 33denotes a p base layer, reference numeral 34 denotes an n⁺ emitterregion, reference numeral 38 denotes an n buffer layer, referencenumeral 39 denotes a p collector layer, reference numeral 41 denotes aninterlayer insulating film, reference numeral 42 denotes a gateelectrode, and reference numeral 43 denotes a gate insulating film.Reference numeral 1 denotes an n⁻ semiconductor substrate which will bean n⁻ drift layer and reference numeral 23 denotes an interface betweenthe n⁻ drift layer and the p base layer 33.

As described above, according to Embodiment 8, it is possible to obtainthe same effect as that in Embodiment 1.

As described above, in the invention, when the invention is applied tothe IGBT, a p⁺ collector region may be formed by the implantation ofp-type impurity ions, such as boron (B) ions, and a thermal activationprocess, instead of the process of forming the n⁺ cathode layer usingthe implantation of n-type impurity ions and thermal activation. In theabove-described embodiments, the silicon substrate is used as thesemiconductor substrate. However, a SiC (silicon carbide) substrate or aGaN (gallium nitride) substrate may be used. In this case, the sameeffect as described above is expected.

As described above, the semiconductor device and the method forproducing the semiconductor device according to the invention are usefulas power semiconductor devices used for power conversion apparatusessuch as converters or inverters.

Thus, a semiconductor device and method for producing it have beendescribed according to the present invention. Many modifications andvariations may be made to the techniques and structures described andillustrated herein without departing from the spirit and scope of theinvention. Accordingly, it should be understood that the methods anddevices described herein are illustrative only and are not limiting uponthe scope of the invention.

REFERENCE SIGNS LIST

-   -   1 thin n⁻ semiconductor substrate    -   1 a rear surface of thin n⁻ semiconductor substrate 1    -   1 b front surface of thin n⁻ semiconductor substrate 1    -   2 p-type anode region    -   3 n⁺ cathode layer    -   4 junction edge termination structure    -   5 anode electrode    -   6 cathode electrode    -   8 insulating film    -   9 n-type field stop layer    -   11 electron beam irradiation    -   12 crystal defect generated by electron beam irradiation 11    -   13 proton implantation    -   14 hydrogen atom    -   15 crystal defect generated by proton implantation 13    -   18 donor layer    -   20 thick n⁻ semiconductor substrate    -   20 a front surface of thick n⁻ semiconductor substrate 20    -   20 b rear surface of thick n⁻ semiconductor substrate 20    -   21 grinding    -   100 diode

What is claimed is:
 1. A semiconductor device, comprising: asemiconductor substrate having a first and a second side, and comprisinga donor layer with a doping concentration higher than the dopingconcentration of the semiconductor substrate, with a dopingconcentration profile in a depth direction from the first side to thesecond side of the substrate; at least a first peak in said dopingconcentration profile, situated at a first distance Rp1 from the firstside of said substrate; and a region adjacent to Rp1 and extending inthe depth direction from Rp1 toward the first side, which has a dopingconcentration which is substantially uniform.
 2. A semiconductor deviceaccording to claim 1, further comprising a second peak in dopingconcentration, situated at a second distance Rp2 from the first side ofsaid substrate, wherein said second distance Rp2 is less than said firstdistance Rp1.
 3. A semiconductor device according to claim 2, furthercomprising a region adjacent to Rp2 and extending in the depth directionfrom Rp2 toward the first side of the substrate, which has a dopingconcentration which is substantially uniform.
 4. A semiconductor deviceaccording to claim 2, wherein Rp2<0.5Rp1.
 5. A semiconductor deviceaccording to claim 1, wherein Rp1 is 15 μm or more from the second sideof the said substrate.
 6. A semiconductor device according to claim 2,wherein doping concentration in a region between Rp1 and Rp2 issubstantially uniform.
 7. A semiconductor device according to claim 6,wherein doping concentration in the region between Rp1 and Rp2 is atleast 10¹⁴/cm³.
 8. A semiconductor device according to claim 1, furthercomprising a region between Rp2 and the first side of the substrate,wherein doping concentration is substantially uniform.
 9. Asemiconductor device according to claim 8, wherein doping concentrationin the region between Rp2 and the first side of the substrate is atleast 10¹⁴/cm³.
 10. A semiconductor device according to claim 8, whereindoping concentration in the region between Rp2 and the first side of thesubstrate is greater than doping concentration in the region between Rp1and Rp2.
 11. A semiconductor device according to claim 2, wherein dopingconcentration at a distance beyond Rp1 in the depth direction issubstantially uniform.
 12. A semiconductor device according to claim 11,wherein doping concentration at the distance beyond Rp1 is less thandoping concentration in the region between Rp1 and Rp2.
 13. Asemiconductor device according to claim 11, wherein doping concentrationat a distance greater than Rp1 in the depth direction is at least10¹⁴/cm³.
 14. A semiconductor device according to claim 1, whereinhydrogen terminates dangling bonds in the region between Rp1 and Rp2.15. A semiconductor device according to claim 1, wherein hydrogenterminates dangling bonds in the region between Rp2 and the first sideof the substrate.
 16. A semiconductor device according to claim 1,wherein the region between Rp1 and Rp2 is a donor of avacancy-oxygen-hydrogen complex defect.
 17. A semiconductor deviceaccording to claim 1, wherein the region between Rp2 and the first sideof the substrate is a donor of a vacancy-oxygen-hydrogen complex defect.18. A semiconductor device according to claim 1, wherein the first peakin said doping concentration profile is situated at a first distance Rp1which is approximately 50 to 60 μm from the first side of the substrate.19. A semiconductor device, comprising: a semiconductor substrate havinga first and a second side, and comprising a donor layer with a dopingconcentration higher than the doping concentration of the semiconductorsubstrate, with a doping concentration profile in a depth direction fromthe first side to the second side of the substrate; at least a firstpeak in said doping concentration profile, situated at a first distanceRp1 from the first side of said substrate; and a region adjacent to Rp1and extending in the depth direction from Rp1 toward the first side,which has a doping concentration of the semiconductor substrate orhigher.
 20. A semiconductor device according to claim 19, furthercomprising a second peak in doping concentration, situated at a seconddistance Rp2 from the first side of said substrate, wherein said seconddistance Rp2 is less than said first distance Rp1.
 21. A semiconductordevice according to claim 20, wherein doping concentration in the regionbetween Rp1 and Rp2 is at least 10¹⁴/cm³.
 22. A semiconductor deviceaccording to claim 20, wherein Rp2<0.5Rp1.
 23. A semiconductor deviceaccording to claim 22, wherein Rp1 is 15 μm or more from the second sideof the said substrate.
 24. A semiconductor device according to claim 19,further comprising a region adjacent to Rp2 and extending in the depthdirection from Rp2 toward the first side of the substrate, which has adoping concentration which has a doping concentration of thesemiconductor substrate or higher.
 25. A semiconductor device,comprising: a semiconductor substrate having a first and a second side,and comprising a donor layer with a doping concentration higher than thedoping concentration of the semiconductor substrate, with a dopingconcentration profile in a depth direction from the first side to thesecond side of the substrate; at least a first peak in said dopingconcentration profile, situated at a first distance Rp1 from the firstside of said substrate; a region adjacent to Rp1 and extending in thedepth direction from Rp1 toward the first side, which has a dopingconcentration of at least 10¹⁴/cm³, and a second peak in dopingconcentration, situated at a second distance Rp2 from the first side ofsaid substrate, wherein said second distance Rp2 is less than said firstdistance Rp1.
 26. A semiconductor device according to claim 25, furthercomprising a region adjacent to Rp2 and extending in the depth directionfrom Rp2 toward the first side of the substrate, which has a dopingconcentration of at least 10¹⁴/cm³.